Most of the time, engineers require the defects’ cross sectional dimension with the wall thickness to calculate the percent remaining and joint efficiency.

It is possible to plot a reflector, and give its location and cross sectional dimension as a percentage.

The half skip sound path distance (shown in blue in Figure 60) is independent of the angle or radius, and the position on the screen and beam path travelled is 100% wall thickness, while the 2nd leg shown in red is 100% to 200%

This will show on the screen with sufficient gain and is known as
Inside Diameter (ID) roll regardless if it is plate or pipe. Some technicians call it “Back wall”.

Therefore if the bottom corner reflector (or ID roll) comes up on the screen at 40mm that is equal to 100% and the top will be at 80mm and 200%. An indication coming up at 30mm will be 75% down from the top surface.

For ease of interpretation, you may adjust the range control so that the ID roll is placed at mid screen.

Another method of using the ID roll is to move the ID roll to represent the direct thickness of the test specimen, or multiples there of.

For example:
The ID roll of a pipe with a 0.450” wall thickness, to be placed at 4.5, or the ID roll from a 20mm thick shell wall to be placed at 20mm or 40mm or 50mm on the time base.